Fix the warnings of CFConv

[raimis]

It also reports what look like some genuine errors in the code:

/Users/runner/work/NNPOps/NNPOps/src/pytorch/CFConv.cpp:101:29: warning: equality comparison result unused [-Wunused-comparison]
                activation_ == ::CFConv::ShiftedSoftplus;
                ~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/Users/runner/work/NNPOps/NNPOps/src/pytorch/CFConv.cpp:101:29: note: use '=' to turn this equality comparison into an assignment
                activation_ == ::CFConv::ShiftedSoftplus;
                            ^~
                            =
/Users/runner/work/NNPOps/NNPOps/src/pytorch/CFConv.cpp:103:29: warning: equality comparison result unused [-Wunused-comparison]
                activation_ == ::CFConv::Tanh;
                ~~~~~~~~~~~~^~~~~~~~~~~~~~~~~
/Users/runner/work/NNPOps/NNPOps/src/pytorch/CFConv.cpp:103:29: note: use '=' to turn this equality comparison into an assignment
                activation_ == ::CFConv::Tanh;
                            ^~
                            =

Originally posted by @peastman in #63 (comment)